When data is transmitted in a network, errors may arise.
By way of in no way limiting example, the data transmitted over a packet switched communication network is organized in packets of binary data and the errors to which these communications are generally subject are errors on the data constituting a network packet, as well as losses of complete network packets.
These errors on the data constituting a packet are missing or erroneous data in the network packet, for example an inversion of the value of one or more binary data items or the loss of one or more data items.
These errors are typically due to the nature of the transmission channel.
The loss of network packets is typically due to congestion on the network, as well as to the rejection of corrupted network packets, that is to say those comprising one or more errors and/or omissions.
The bit errors may be indicated by:                a lower network layer, or        correcting codes that were incapable of correcting an error but were able at least to discern its position.        
Thus, when a stream of coded data containing the payload of at least one network packet arrives at a decoder in order to be decoded, and this data stream includes an error, the decoder may suffer desynchronization when this error is read.
Some decoders, when they detect that an error has occurred, go into resynchronization mode, that is to say they seek a synchronization marker indicating the start of a data sequence or the start of a packet of coded data, for example video data.
Nevertheless, this error being detected a certain length of time after it occurred, the decoder continues to read the data of the data stream. This data is therefore not decoded correctly, given that the decoder is desynchronized after the reading of the error.
In addition, between the moment when the decoder reads the error (the moment when it desynchronizes) and the moment when the decoder knows that an error has occurred (the moment when it goes into resynchronization mode), the decoder, desynchronized, may interpret the start of a packet of coded data erroneously. Thus the following data in this packet of coded data is interpreted erroneously.
In both cases, data that was valid (that is to say that did not contain any erroneous data) is wrongly decoded.
Thus, in addition to the presence of errors in the data stream for the reasons disclosed above, these errors may propagate in the data stream, thus causing additional errors.
The ISO/IEC standard 14496-2 “Information technology—coding of audio-visual objects—part 2: Visual” describes the attribution of a predetermined value to a data item belonging to the data stream in order to indicate, within the data stream, the presence of an error.
This data item indicating the presence of an error consists of a bit situated at the start of the data stream corresponding to an image.
In the MPEG-4 standard, an image is called VOP (from the English “Video Object Plane”). More particularly, the bit is situated in an information unit called GOV or GOP (from the English “Group of Video Object Plane”) situated before an image or VOP of the I-VOP type (from the English “Intra coded VOP”. This type of VOP contains all the information necessary for being decoded by the decoder.
The images or VOP can also be of the P-VOP type (from the English “Predictive coded VOP”). This type of VOP requires a reference image for its decoding.
The images or VOP can also be of the B-VOP type (from the English “Bidirectionally-predictive coded VOP”), requiring two reference images, including at least one of the P-VOP type. In the context of the use of this bit, an image of the B-VOP type uses as references an image of the I-VOP type (the preceding one in the data stream) and an image of the P-VOP type (the following one in the data stream).
When the bit indicating the presence of an error has a value “1”, the decoder is warned of the fact that the first B-VOP or the first sequence of consecutive B-VOPs situated after the I-VOP that is situated at the start of the data stream will not be correctly decoded, since the P-VOP that should follow the B-VOP or B-VOPs is not present in the data stream.
The decoder thus takes the decision not to display these B-VOPs or images.
Thus, by means of a bit situated at the start of the data stream, the decoder is warned of the existence of an error.
Nevertheless the possibility of warning the decoder of the presence of an error by means of this bit is limited to the type of error described above. The bit cannot be used to indicate other types of error.
Moreover, it is situated at a fixed position in the data stream, which does not make it possible to locate the errors.
The document U.S. Pat. No. 5,778,191 proposes to insert synchronization points in fixed positions of the bit stream produced by a coder, in order to improve the detection of errors and the resynchronization of a decoder.
This solution, in addition to not making it possible to locate the exact position of the errors, has an influence on the compression format, and this involves incompatibilities of the formats with certain standards.